1. Field of the Invention
The present invention relates to a liquid-filled vibration isolator, and more particularly to a liquid-filled vibration isolator mounted on a vehicle between a body and, for example, a sub-frame and adapted to reduce oblique vibration input.
2. Description of the Related Art
Conventionally, such a liquid-filled vibration isolator includes, as shown in FIGS. 11 to 13, an inner cylindrical metal member 1, a cylindrical intermediate metal member 2, and a cylindrical rubber elastic-body 3. The intermediate metal member 2 is disposed coaxially outside the inner cylindrical metal member 1 with a space formed therebetween, and includes a pair of ring portions 2a and a pair of connection portions 2b. The ring portions 2a are disposed at corresponding axial ends of the intermediate metal member 2. The connection portions 2b are disposed at radially opposite positions and connect the ring portions 2a to thereby define a pair of window portions 2c in cooperation with the ring portions 2a. The cylindrical rubber elastic-body 3 includes a pair of side wall portions 3a, arm portions 3b, an thin-walled seal portion 3c, and a pair of cavity portions. The side wall portions 3a radially connect the respective ring portions 2a and the inner cylindrical metal member 1. The arm portions 3b extend axially between the side wall portions 3a and inside the connection portions 2b so as to connect the connection portions 2b and the inner cylindrical metal member 1. The thin-walled seal portion 3c is disposed outside the ring portions 2a. The cavity portions are defined by the side wall portions 3a and the arm portions 3b and open at the corresponding window portions 2c. 
An outer cylindrical metal member 5 is disposed coaxially outside the intermediate metal member 2 and is fixedly attached to the intermediate metal member 2 via the seal portion 3c through drawing to thereby establish liquid tightness against the intermediate metal member 2. The thus-sealed cavity portions form a pair of liquid chambers 6. An orifice path 7 is formed between and extends along the outer cylindrical metal member 5 and the connection portion 2b so as to establish communication between the two liquid chambers 6.
The thus-configured liquid-filled vibration isolator is mounted on the body of a vehicle in, for example, the following manner. While the two liquid chambers 6 are oriented along the front-rear direction of the vehicle, the inner cylindrical metal member 1 is perpendicularly connected to a support member (not shown) of the vehicle body, and the outer cylindrical metal member 5 is connected to a rear sub-frame (not shown). The liquid-filled vibration isolator receives vibration input P that is imposed obliquely by the sub-frame according to acceleration/deceleration of the vehicle, and functions to damp vibration input P.
However, in the conventional liquid-filled vibration isolator, the side wall portions 3a of the rubber elastic-body 3 extend radially so as to connect the ring portions 2a and the inner cylindrical metal member 1; thus, the free length of the side wall portions 3a is short. As a result, the tensile strain of the side wall portions 3a derived from vibration input P becomes large (for example, the FEM (Finite Element Modeling) strain is about 130% to 215%), thereby impairing durability of the side wall portions 3a. 
In order to cope with the above problem, a liquid-filled vibration isolator as shown in FIG. 14 is provided. As shown in FIG. 14, in radial connection of the ring portions 2a and the inner cylindrical metal member 1 by means of side wall portions 3h of the rubber elastic-body 3, the position of connection between the inner cylindrical metal member 1 and each side wall portion 3h is axially shifted toward the axial center of the inner cylindrical metal member 1 such that the side wall portions 3h assume an S-shaped cross section to thereby lengthen the free length thereof. The free length of the side wall portions 3h is thus lengthened in an attempt to ease tensile strain derived from vibration input P in order to enhance the durability of the side wall portions 3h. However, even when the free length of the side wall portions 3h is increased, the tensile strain produced in the side wall portions 3h due to vibration input from an oblique direction is still large, so that the FEM strain thereof becomes 110% or higher, failing to achieve a target FEM strain of 80%. Thus, the liquid-filled vibration isolator still fails to exhibit sufficient durability of the rubber elastic-body 3.
An object of the present invention is to solve the above-mentioned problem in the conventional liquid-filled vibration isolator and to provide a liquid-filled vibration isolator capable of reducing the tensile strain of side wall portions of a rubber elastic-body produced due to vibrations which are input from an oblique direction according to acceleration/deceleration of a vehicle, to thereby enhance the durability of the rubber elastic-body.
To achieve the above object, the present invention provides a liquid-filled vibration isolator comprising an inner cylindrical metal member; a cylindrical intermediate metal member disposed outside the inner cylindrical metal member with a space formed between the inner cylindrical metal member and the intermediate metal member, the intermediate metal member comprising a pair of ring portions disposed at corresponding axial ends thereof, a pair of connection portions disposed at radially opposite positions and integrally connecting the ring portions, and a pair of window portions defined by the ring portions and the connection portions; a rubber elastic-body comprising a pair of inclined side wall portions connecting the ring portions of the intermediate metal member and an axially central portion of the inner cylindrical metal member along a circumferential direction, arm portions axially connecting the side wall portions and connecting the connection portions and the inner cylindrical metal member, and a pair of cavity portions defined by the side wall portions and the arm portions and opening at the corresponding window portions; an outer cylindrical metal member fixedly disposed outside the intermediate metal member in a substantially coaxial manner and sealed in a liquid-tight manner against the intermediate metal member by means of a rubber seal portion, to thereby define a pair of liquid chambers in the corresponding cavity portions; and an orifice path formed between and extending along the outer cylindrical metal member and the connection portion of the intermediate metal member so as to establish communication between the liquid chambers. The inner cylindrical metal member is oriented in a vertical direction and fixedly attached to a member of a vehicle body with the liquid chambers being oriented in the front-rear direction of a vehicle, and the outer cylindrical metal member is fixedly attached to a counterpart member. A first pair of inclined side wall portions are formed at one of radially opposite circumferential positions between the connection portions so as to be located on the front side of the inner cylindrical metal member with respect to the front-rear direction of the vehicle, and a second pair of inclined side wall portions are formed at the other of the radially opposite circumferential positions so as to be located on the rear side of the inner cylindrical metal member with respect to the front-rear direction of the vehicle. Among the first and second pairs of inclined side wall portions, two inclined side wall portions located diagonally opposite each other with respect to the inner cylindrical metal member are longer than the remaining two side wall portions.
Preferably, the diagonal direction along which the longer side wall portions extend coincides with the direction of vibration input.
In the present invention, the inner cylindrical metal member is oriented in a vertical direction and fixedly attached to a member of a vehicle body in a sate in which the pair of liquid chambers are oriented in the front-rear direction of a vehicle, and among the side wall portions of the rubber elastic-body, the longer side wall portions located diagonally opposite each other with respect to the inner cylindrical metal member are oriented in the front-rear direction and in a vibration input direction. In this state, the outer cylindrical metal member is fixedly attached to a counterpart member, which receives vibrations generated in an oblique direction with respect to the front-rear direction of the vehicle. Since the side wall portions for coping with vibrations which are input from an oblique direction according to acceleration/deceleration of the vehicle have increased free lengths, the tensile strain of the side wall portions of the rubber elastic-body derived from the vibration input can be reduced.
As described above, among the side wall portions of the rubber elastic-body of the liquid-filled vibration isolator, the longer side wall portions located diagonally opposite each other with respect to the inner cylindrical metal member are disposed in order to cope with vibrations which are input from an oblique direction according to acceleration/deceleration of the vehicle, the tensile strain of the side wall portions of the rubber elastic-body derived from the vibration input can be reduced, thereby enhancing the durability of the rubber elastic-body.
Further preferably, the liquid-filled vibration isolator further comprises a pair of stoppers fixedly attached to the outer cylindrical surface of the inner cylindrical metal member in such a manner as to project into the corresponding liquid chambers and in such a manner as to be vertically biased toward opposite directions with respect to the center of the inner cylindrical metal member.
Still further preferably, the side wall portions each assume an S-shaped axial cross section to thereby lengthen the side wall portions.